This invention relates to a steel for machine structural use, which is to be subjected to machining for use as industrial machinery or automotive parts, among others. More particularly, the invention relates to a steel for machine structural use excellent in chip disposability and effective in prolonging the cutting tool life (hereinafter referred to as xe2x80x9ctool life improvementxe2x80x9d).
Among the steels for machine structural use, which are used as industrial machinery or automotive parts, among others, there are steels for machine structural use as defined in the Japanese Industrial Standard JIS G 4051, and such alloy steels as nickel-chromium steels according to JIS G 4102, nickel-chromium-molybdenum steels according to JIS C 4103, chromium steels according to JIS G 4104 and manganese and manganese-chromium steels for machine structural use according to JIS G 4106. Also in use are steels improved in hardenability by modifying the amount of addition of the specified components of these steels or by adding B (boron) or the like and/or improved in metallurgical structure by addition of Ti, Nb: V and/or the like.
In many cases, these steels are subjected, after rolling or after further forging or other working, to machining to desired forms or shapes, followed by heat treatment according to the required characteristics, to give final products. For improving the productivity in this machining step, it is strongly desired that the steels be excellent in machinability. Good machinability means that the period between exchanges of tools for use in machining due to wear is long, that is, that the tool life is long, that chips generated during machining can be finely torn and separated, that the cutting force is not so great, and that good machined or ground surfaces can be obtained.
With the advancement in automation of machining, not only the tool life but also the separability of chips, namely xe2x80x9cchip disposabilityxe2x80x9d, becomes very important. Since the tool life is influenced by the characteristics of the material steel as well as the performance characteristics of the tool, tool selection is also important. On the contrary, good chip disposability means that chips generated during machining are finely torn or divided and separated but will not entwine the tool. The chip disposability greatly depends on the characteristics of the material steel. For improving the machinability of steel, it is very important to improve this chip disposability.
The machinability of steel can be improved by addition of Pb. However, addition of Pb not only increases the cost of steel but also may lead to environmental contamination. Therefore, investigations have been carried out in search of technologies of improving the machinability of steel without adding Pb. A typical one is the technology of improving the machinability by utilizing MnS inclusions. This technology has been studied in various aspects and already put to practical use.
Thus, for example, the steel disclosed in Japanese Patent Publication (JP Kokoku) H05-15777 contains Mnxe2x80x94Caxe2x80x94S type inclusions with a Ca content of 3xe2x80x9455% as uniformly dispersed therein. As for their sizes, the major axis L is not longer than 20 xcexcm and the ratio thereof to the minor axis W (L/W) is not more than 3. In this steel, however, individual sulfide inclusions become coarse, hence the number of sulfide inclusions at the same S concentration decreases. Therefore, the improvement in chip disposability is not entirely satisfactory. In addition, because the steel is Al-killed steel, even after treatment with Ca, the oxide inclusions are of the CaOxe2x80x94Al2O3 type, hence the improving effects on the machinability such as tool life are not very satisfactory. When an attempt is made to disperse a large number of sulfide inclusions containing a high concentration of CaS by increasing the S concentration, addition of a large amount of Ca is required, and this disadvantageously causes an increase in cost.
Laid-open Japanese Patent Application (JP Kokai) 2001-131684 discloses steels for machine structural use, in which manganese sulfide-based inclusions have an average oxygen content of not more than 10%. The steels have the following principal composition, in % by mass: C: 0.05-0.7%, Si: not more than 2.5%, Mn: 0.1-3.0%, Al: not more than 0.1%, S: 0.003-0.2%, N: 0.002-0.025%, and O (oxygen): not more than 0.003%, with the balance being Fe. In addition to these components, the steels may contain not more than 0.01%, in total, of one or more elements selected from the group consisting of rare earth elements, Ca and Mg.
However, the steels according to the invention disclosed in JP Kokai 2001-131684, as described in the example section thereof, contain not less than 0.018% of Al used as a deoxidizer clement so that the average oxygen concentration in sulfides may be reduced to 10% or less for obtaining such a sulfide form as effective in improving the chip disposability. In such a case, the oxides in steel are mainly hard Al2O3 type oxides, and the tool life is improved only to an unsatisfactory extent. Thus, the invention disclosed in the above-cited publication is not an invention made in an attempt to improve the chip disposability and, at the same time, improve the tool life.
In JP Kokai 2000-34538, there is disclosed a steel for machine structural use which contains C, Si, Mn, P, S, Al, Ca and N each in a specified amount and is excellent in machinability in turning. This steel has the following characteristic features. Namely, the following two relations are satisfied:
A/(A+B+C)xe2x89xa60.3 and B/(A+B+C)xe2x89xa70.1
wherein A is the area percentage of sulfide inclusions having a Ca content exceeding 40% relative to the total area of an investigation field of view, B is the area percentage of sulfide inclusions having a Ca content of 0.3-40% relative to the total area of the investigation field view, and C is the area percentage of sulfide inclusions having a Ca content of less than 0.3% relative to the total area of the investigation field of view. The steel of JP Kokai 2000-34538 is characterized by increasing sulfide containing 0.3-40% of Ca. However, increase of such sulfide of high Ca content makes the sulfide coarse and makes improvement of chip disposability difficult.
JP Kokai 2000-282169 discloses a steel, which contains C, Si, Mn, P and S and further contains one or more elements selected from among Zr, Te, Ca and Mg and satisfies the conditions: Alxe2x89xa60.01%, total Oxe2x89xa60.2% and total Nxe2x89xa60.02%. This steel is excellent in forgeability owing to spheroidizing of sulfide inclusions and has good machinability. Thus, on the premise that Ca is added, it is intended that Ca solutes in MnS and lowers the deforming ability of MnS for spheroidizing the same in this steel. In this case, however, individual sulfide inclusions become coarse, whereby that sulfide morphology suited for providing good chip disposability cannot he obtained, hence the improvement in chip disposability is not yet satisfactory.
All the steels disclosed in the above mentioned publications may contain Ca and are improved primarily in machinability. However, it cannot be said that sufficient considerations have been given to the level of addition of Ca, the timing of addition thereof and the dissolved oxygen content in the steel. Thus, they are not satisfactorily improved simultaneously in chip disposability and in tool life.
It is an object of the present invention to provide a steel for machine structural use, which is improved in machinability, especially in chip disposability and, at the same time, can prolong the tool life, without containing Pb.
It is well known that the machinability of steel is greatly influenced by the state of sulfide and/or oxide inclusions in the steel. For improving the machinability of Pb-free steels for machine structural use, the present inventors made close investigations concerning the relationship between the form and distribution of inclusions in the steels and the machinability thereof, and studied the investigation results. The inventors paid attention to the effects of Ca and Ti, in particular, and investigated the steelmaking conditions as well. In the process of these investigations and studies, the inventors reveal the following remarkable facts.
Ca strongly binds to S and alters the form of sulfide inclusions, mainly MnS, and shows a large bonding strength with oxygen, leading to stable oxide formation.
When Ca is added without paying any attention to the steelmaking conditions, CaS or Ca-based oxides formed in the molten steel serve as nuclei for the formation of MnS grains and the number of sulfide inclusions having a Ca content of not less than 1% increases. It was found, however, that when, in adding Ca, the steelmaking conditions, such as the level of addition thereof, the dissolved oxygen level and the timing of addition of Ca, are appropriately selected, sulfide inclusions mainly composed of MnS not containing Ca are formed in large amounts. Further, it was revealed that the chip disposability of steel becomes improved only in such case.
There are two type inclusions, i.e., sulfide type one and oxide type one. Since minute inclusions such as precipitates are not effective in machinability improvement, it was decided that the size of inclusion should be evaluated in terms of the diameter of a circle equivalent in area to the inclusion in the observation field of view, and investigations were made regarding inclusions greater in such diameter than a certain level.
As a result, it was found that when the number of almost Ca-free sulfide exceeds 90%, or, in other words, when the number of Ca-containing MnS type inclusions is less than 10%, particularly good chip disposability can be obtained.
When compared at the same S content level, steels, in which a large number of small sulfide inclusions are present, are superior in chip disposability to steels in which a small number of coarse sulfide inclusions are present. When an increased amount of sulfides containing Ca as solid solution is present in the molten steel or at the initial stage of solidification, they serve as nuclei for crystallization of MnS, giving coarse sulfide inclusions. Therefore, at the same S concentration, the number of dispersed sulfide inclusions decreases and fine sulfide inclusions are hardly formed. When, on the other hand, the amount of sulfide inclusions containing Ca as solid solution is small, the sulfide inclusions mostly form a large number of fine sulfide inclusions.
A chip generated during machining is torn or separated when stress is concentrated on inclusions in the deformed steel chip, resulting in crack formation and propagation. Ca-free MnS type inclusions tend to be deformed in the direction of working, for example rolling, and many of them have an elongated form. When large elongated inclusions are present, the anisotropy in mechanical properties of a steel material increases and, in addition, the number of inclusions to serve as points for stress concentration and starting points of chipping decreases, hence no good chip disposability can be obtained. On the other hand, when there are a large number of small inclusions, the number of crack starting points in chips subjected to deformation during machining increases and, further, stress is concentrated on the inclusions and crack propagation becomes readily promotable thereby. This is presumably the cause of improvement in chip disposability.
The tool life is greatly influenced by the composition of oxides contained in the steel. When Ca is added to convert oxides to low-melting oxides, the tool life is markedly prolonged. Therefore, treatment with Ca is essential. For attaining both the above-mentioned sulfide control and oxide control simultaneously, the steelmaking conditions before and after Ca treatment were further examined in detail. As a result, the following facts were revealed. It becomes possible to control the oxide inclusions so that they may be composed of CaOxe2x80x94Al2O3xe2x80x94SiO2xe2x80x94TiO2 as main constituents even within the same composition range, by restricting the contents of those components showing a high level of interaction with oxygen in steel, such as C, Si and Mn, causing S to be contained at a specific level, reducing Al as far as possible, adding Ti and Ca each at an appropriate addition level and at an appropriate time and adjusting the level of dissolved oxygen. These oxide inclusions are low in melting point and soft and are presumably effective not only in tool life improvement owing to Ca and Ti contained therein but also in producing starting points for cracking in chips and promoting crack propagation.
The influences of the compositions with respect to C, Si, Mn and so on and of the contents of Cr, Ni, Mo, B, Nb, V and other elements, which are added for improving the strength, hardenability, metallurgical structure and other properties of steels for machine structural use, on the improvement in chip disposability and tool life as attainable by such forms of sulfide inclusions and oxide inclusions were examined. As a result, it could be confirmed that while these elements may improve the strength, hardenability and other mechanical characteristics of steels, the effect of the invention, namely the improvement in machinability with the same composition can be produced in the same manner.
Accordingly, the present inventors further established the limits to the chemical composition and to the states or forms of inclusions and, as a result, have completed the present invention. The gist of the invention is as follows.
(1) A steel for machine structural use consisting of, in percent by mass, C: 0.1-0.6%, Si: 0.01-2.0%, Mn: 0.2-2.0%, P: not more than 0.1%, S: 0.005-0.2%, Al: not more than 0.009%, Ti: not less than 0.001% but less than 0.04%, Ca: 0.0001-0.01%, O (oxygen): 0.001-0.01%, and N: not more than 0.02%, and the balance Fe and impurities, and satisfying the following relations (1) to (3) with respect to the inclusions in the steel:
n0/S (%)xe2x89xa72500xe2x80x83xe2x80x83(1)
n1/n0xe2x89xa60.1xe2x80x83xe2x80x83(2)
n2xe2x89xa710xe2x80x83xe2x80x83(3)
wherein n0, n1 and n2 are defined as follows:
n0: total number of sulfide, having a circle equivalent diameter of not less than 1 xcexcm, per mm2 of a cross section parallel to the direction of rolling, number/mm2;
n1: number of MnS, having a circle equivalent diameter of not less than 1 xcexcm and containing not less than 1.0% of Ca, per mm2 of a cross section parallel to the direction of rolling, number/mm2;
n2: number, per mm2 of a cross section parallel to the direction of rolling, of oxide inclusions having a composition comprising CaOxe2x80x94Al2O3xe2x80x94Si2xe2x80x94TiO2 and impurities, with CaO: 5-60%, Al2O3: 5-60%, SiO2: 10-80% and TiO2: 0.1-40% when the sum of CaO, Al2O3, Si2O2 and TiO2 is taken as 100% by mass, and having a circle equivalent diameter of not less than 1 xcexcm, number/mm2.
(2) A steel for machine structural use which comprises, in addition to the components mentioned above in (1), one or more elements selected from the first group and/or second group shown below and satisfies the relations (1), (2) and (3) given above.
First group:
Cr: 0.02-2.5%, V: 0.05-0.5%, Mo: 0.05-1.0%, Nb: 0.005-0.1%, Cu: 0.02-1.0% and Ni: 0.05-2.0%;
Second group:
Se: 0.0005-0.01%, Te: 0.0005-0.01%, Bi: 0.05-0.3% and rare earth elements: 0.0001-0.0020%.